Partial conversion or “Mild” hydrocracking has been utilized by refiners for many years to produce incremental middle distillate yields while upgrading feedstock for fluid catalytic cracking (FCC). Initially, specialized catalysts were adapted to the low or moderate pressure conditions in FCC feed desulfurizers to achieve 20 to 30 percent conversion of heavy gas oils to diesel and lighter products. The combination of low pressure and high temperatures used to achieve hydro-conversion conditions typically resulted in heavy, high aromatic products with low cetane quality. The promulgation of new specifications for both gasoline and diesel products has put pressure on such processes to make lighter, lower sulfur products that can fit into the refinery ultra low sulfur diesel and gasoline (ULSD and ULSG) pools. The continued growth in middle distillate fuel demand compared to gasoline has re-focused attention on hydrocracking and particularly on partial conversion hydrocracking as a key process option for adapting to the modern clean fuels environment.
New specifications in both the U.S. and E.U. have mandated dramatic reductions in both diesel and gasoline sulfur levels. It is now clear that lower sulfur levels in these products provide substantial benefits in terms of decreased tail pipe emissions from automobiles and trucks. Pipeline transportation of both low sulfur and high sulfur distillate grades is still a work in progress. Recent studies in the U.S. indicate that as much as 10% of ultra low sulfur diesel (ULSD) will be downgraded by common pipeline transportation, and some carriers are requiring that ULSD be no more than 5 wppm sulfur at the refinery boundary. The environmental benefits and product transportation logistics make it certain that there will be continued pressure to force all fuels into the ultra low sulfur category.
Conventional partial conversion units utilised in many refineries around the world have been designed for pressure levels in the 50 to 100 barg range depending on feed quality and cycle life objectives. They have been designed to achieve 20% to 30% net conversion of heavy vacuum gas oil and total sulfur removal of about 95% to yield FCC feed suitable for making low sulfur gasoline. The process configuration has evolved to include hot high pressure separators for better heat integration and amine absorbers to mitigate the effects of very high recycle gas hydrogen sulfide content.
One significant shortcoming of this technology has been the inability to have independent control of hydro-conversion and hydro-desulfurization reaction severity. While the diesel product sulfur can be decreased to a large extent by applying more hydrotreating catalyst and achieving deeper HDS severity, the only real option for improving density and cetane quality is to increase reactor operating pressure or to increase hydrocracking severity.
Large increases in reactor pressure can raise chemical hydrogen consumption by 70% to 100%. The high capital and operating cost associated with such large increases in hydrogen consumption is a significant disadvantage for utilizing high pressure designs to achieve product uplift.
WO patent application No. 99/47626 discloses an integrated hydroconversion process comprising hydrocracking a combined refinery and hydrogen stream to form liquid and gaseous components. Unreacted hydrogen from the hydrocracking step is combined with a second refinery stream and hydrotreated. The product is separated into a hydrogen stream and a portion of this stream is recycled to the hydrocracking step. Higher yields of naphtha and diesel and lower yields of fuel oil were obtained. However, this process has the disadvantage of requiring a feedstock with relatively low nitrogen, sulfur and aromatics content. This implies, in many cases, that the feedstock needs to be pre-treated prior to the disclosed process.
U.S. Pat. No. 6,294,079 discloses an integrated low conversion process comprising separating the effluent from a hydrotreating step into three fractions: a light fraction, an intermediate fraction and a heavy fraction. The light fraction and a portion of the intermediate and heavy fractions are bypassed the hydrocracking zone and sent to a separator. A series of high pressure separators are used. The remaining intermediate and heavy fractions are hydrocracked. FCC feedstock is produced. An augmented separator and other separators are used to separate the hydrotreater effluent into a vapour stream and two liquid streams. Parts of each liquid stream are flow controlled and remixed with the cooled, compressed vapour stream, reheated and hydrocracked at high severity to produce the higher quality middle distillate products. The complex arrangement of multiple separators and the cooling of the vapour stream lead to the use of extra equipment and added cost.
Increasing overall hydrocracking severity is at times not a viable option. When the process objective is to make a required amount of FCC feed, a high conversion leads to the formation of good quality diesel. However, high conversion also results in production of insufficient FCC feed since more diesel is produced.
The objective of this invention is to provide a process and apparatus in which FCC feed is treated to produce ultra low sulfur FCC feed suitable for production of ultra low sulfur gasoline (USLG) not requiring gasoline post treatment.
Another objective of this invention is to provide a process and apparatus for producing diesel with an ultra low sulfur content and substantially improved ignition quality as measured by cetane number, cetane index, aromatics content and density.
A further objective of this invention is to provide a simple apparatus for carrying out the process of the invention.